Experiment 9: Dehydration of 2-Butanol via Gas Collection & Gas Chromatography Analysis

Question 1

If a compound contains an isolated ethyl group (CH3CH2), which of the following splitting patterns would you expect to observe in the proton NMR spectrum of the compound?

Answer 1

quartet, triplet

Question 2

In today’s experiment, butan-2-ol is dehydrated to yield 3 isomeric butenes (1-butene, cis-2-butene, trans-2-butene). How will the crude mixture of butene products be isolated?

Answer 2

We will collect them as a gas mixture over water.

Question 3

! In today’s experiment, 2-butanol is dehydrated to yield 3 isomeric butenes. If 60 mmols of 2-butanol is used and the product mixture contains 10.0 mmol of trans-2-butene, 30.0 mmol of 1-butene, and 20.0 mmol of cis-2-butene, what is the mol fraction of each in the product mixture?

Answer 3

Trans-2-butene: 0.167
1-butene: 0.500
Cis-2-butene: 0.333

Question 4

At the end of the reaction, there is a caution noted in the procedure. Do not remove the reservoir or allow the open end to break the surface of the water. Why?

Answer 4

The product will escape into the atmosphere. Goodbye yield.

Question 5

diagram NMR question of 5 & 7 of Nick’s quiz

Answer 5

see

Question 6

If Proton A is closer to an electronegative element than Proton B, Proton A will be more (shielded/deshielded) than Proton B and appear more (upfield/downfield) from Proton B in an 1H NMR spectrum.

Answer 6

deshielded, downfield

Question 7

! What is the relationship between 1-butene and cis-2-butene?

Answer 7

constitutional isomers

Question 8

What is the role of sulfuric acid in the acid-catalyzed dehydration of 2-butanol?

Answer 8

Sulfuric acid is a catalyst. It makes the hydroxyl group a better leaving group and is not used up in the course of the reaction.

Question 9

In the acid-catalyzed dehydration of 2-butanol, 3 isomeric butenes are formed. How will the mixture of butene products be analyzed to see how much of each butene was formed?

Answer 9

We will do gas chromatography & use the integration of the peaks to calculate the mole fraction of each isomer.

Question 10

If Proton A is closer to an electronegative element than Proton B, Proton A will be more (upfield/downfield) than Proton B and appear at a (higher/lower ppm) from Proton B in an 1H NMR spectrum.

Answer 10

downfield; at a higher ppm

Question 11

What is the relationship between trans-2-butene and cis-2-butene?

Answer 11

diastereomers

Question 12

Several separation techniques are used in organic chemistry such as extraction, TLC, column chromatography, distillation and gas chromatography. TLC and column chromatography often use a liquid as the mobile phase. What is the mobile phase in gas chromatography?

Answer 12

an inert gas such as helium

Question 13

Why is sulfuric acid, H2SO4, used instead of HBr or HCl in the dehydration reaction of 2-butanol?

Answer 13

sulfuric acid is non-nucleophilic and therefore it doesn’t give any substitution products